This application relates to the field of digitized imaging. It is more specifically directed to recovery of an original image from a distorted image replica. It is particularly useful as a defense against negation of imbedded digital image watermarks.
With the development of means of production and circulation of digital images, and the means of imbedding relatively invisible watermarks into digital images ostensibly to convey ownership of the image, there is now financial incentive to attempt to render an imbedded watermark undetectable. Pixel locations of a watermarked image are presumed to correspond to those in an unmarked original image. Generally, the watermark is imbedded by altering only the values of the pixel components of the original image, not their geometric positions. This may be accomplished employing such methods as described in U.S. Pat. No. 5,825,892 which is included herein by reference in its entirety.
Some methods of attacking an imbedded watermark rely on constructing a new image, called a distorted image, that is based on the watermarked image. Pixels in the distorted image are placed at subtly distorted positions relative to those in the watermarked image. Pixel component values in the distorted image are determined by two-dimensional interpolation of component values of enclosing pixel in the watermarked image. No constraints can be placed on the types of pixel position distortion an attacker might choose to use. To those skilled in the art, however, it is obvious that excessive pixel position-distortion will cause the distorted image to be a caricature of the watermarked image, thus diminishing or destroying its economic value. Whether a distortion is excessive is a subjective measure. For a distorted image to be useful it generally requires that whatever linear or nonlinear distortion methods that are used by an attack, have to be used sparingly, and in such a manner as to smoothly varying and relatively small position distortions. This is so as to be essentially unobjectionable and casually unnoticeable to untrained observers. The human visual system, as a qualitative measuring device, can be relied upon to readily detect excessive distortion. It is desirable to have a method of defense without limits on pixel position-distortions placed on the method of attack.
The present invention provides a method of defense against pixel-position distortion types of attacks. The method employs an undistorted reference image relative to which measurements of distortion are made. One aspect of the method employs, a determination of the existence of pixel position distortion; measurement of the amount of pixel position distortion of three or more image features in the distorted image relative to corresponding features in the reference image; based on these measurements, calculation of coefficients of a pixel relocating equation that can specify an approximate position distortion for every pixel in the distorted image. In some embodiments the method also includes a pixel repositioning technique that removes the measured distortion from the distorted image by forming a reoriented image. The reoriented image is, thus, approximately aligned with its corresponding reference image. Once realignment of the distorted image with the reference image is achieved, ordinary watermark extraction methods are used to extract the imbedded watermark from the reoriented image.